Curbs on emissions of gases causing global warming such as carbon dioxide gas have been strengthened against a background of growing environmental protection movements. To address this, the car industry has been actively developing electric vehicles (EVs) and hybrid electric vehicles (HEVs) in place of automobiles using fossil fuels such as gasoline, diesel oil, and natural gas. Batteries used for such EVs and HEVs are generally nickel-hydrogen secondary batteries or lithium ion secondary batteries. In recent years, nonaqueous electrolyte secondary batteries such as a lithium ion secondary battery have been widely used because such a battery is lightweight and has high capacity.
Applications in EVs and HEVs require advancement not only in environmental responsiveness but also in basic performance as an automobile, specifically, running capability such as acceleration performance or hill climbing performance. To satisfy such requirements, not only a mere increase in battery capacity but also a high-power battery is necessary. Secondary batteries used for EVs and HEVs are generally prismatic secondary batteries accommodating a power-generating element within a prismatic outer can. Since a large current flows in the battery when a high-power discharge is performed, the internal resistance of the battery needs to be reduced as much as possible. Therefore, various improvements have also been made to prevent a poor weld between substrate exposed portions of an electrode plate and a collector in a power-generating element of a battery to reduce the internal resistance.
A mechanical fastening method, a welding method, and the like are examples of methods of electrically joining substrate exposed portions of an electrode plate and a collector in a power-generating element to collect power. The welding method is suitable as a collecting method for a battery requiring high power since it is easy to achieve reduction in resistance and temporal change is less likely to occur. Such resistance welding between substrate exposed portions of an electrode plate and a collector of a prismatic secondary battery is performed in a manner below.
For example, in a flattened wound electrode assembly 50 in which a positive electrode plate and a negative electrode plate are wound a plurality of times in a state insulated from each other with a separator therebetween as shown in FIG. 9, a negative electrode collector 52 containing copper or copper alloy is arranged on a surface on one side of a bunch of negative electrode substrate exposed portions 51 containing copper or copper alloy. A negative electrode collector receiving member 53 containing copper or copper alloy is arranged in the same manner on a surface on the other side. The negative electrode collector 52 and the negative electrode collector receiving member 53 are brought into abutment with resistance welding electrodes 54 and 55, respectively, thereby performing resistance welding. This leads to melting of a part of the bunch of negative electrode substrate exposed portions 51 located between a pair of the resistance welding electrodes 54 and 55, and thereby a nugget 56 is formed as appropriate. A favorable electrical conduction is thus achieved between the bunch of negative electrode substrate exposed portions 51 and the negative electrode collector 52 as well as the negative electrode collector receiving member 53.
Positive electrode substrate exposed portions, a positive electrode collector, and a positive electrode collector receiving member (which are omitted in the drawing) have configurations substantially similar to the negative electrode substrate exposed portions 51, the negative electrode collector 52, and the negative electrode collector receiving member 53, respectively, except that the formation material of each is aluminum or aluminum alloy. In this specification, the negative electrode collector or positive electrode collector indicates a member that is directly resistance-welded to the negative electrode substrate exposed portions or positive electrode substrate exposed portions and is used for electrically connecting the negative electrode substrate exposed portions or positive electrode substrate exposed portions to a negative electrode terminal or positive electrode terminal. The negative electrode collector receiving member or positive electrode collector receiving member refers to a member that is directly resistance-welded to the negative electrode substrate exposed portions or positive electrode substrate exposed portions and is used in combination with the negative electrode collector or positive electrode collector.
Patent Document 1 below shows a prismatic secondary battery in which a positive electrode collector or negative electrode collector is welded with a resistance welding method to a substrate exposed portion of a positive electrode plate or negative electrode plate of a flattened wound electrode assembly in which the positive electrode plate and the negative electrode plate are wound a plurality of times in a state insulated from each other with a separator therebetween. A prismatic secondary battery shown in Patent Document 1 below will be described using FIG. 10. FIG. 10A is a vertical sectional view of a terminal portion in the prismatic secondary battery disclosed in Patent Document 1 below, and FIG. 10B is a vertical sectional view of the same at the time of resistance welding.
In a prismatic secondary battery 60 shown in Patent Document 1 below, as shown in FIG. 10A, a wound electrode assembly 62 includes positive electrode substrate exposed portions (omitted in the drawing) and negative electrode substrate exposed portions 61 in which a positive electrode plate and a negative electrode plate are respectively wound in a state insulated from each other with a separator therebetween. The wound electrode assembly 62 is arranged within a prismatic battery outer can 63. For example, the negative electrode substrate exposed portions 61 are bundled in part. A negative electrode collector 64 is resistance-welded to one surface of the bunch of negative electrode substrate exposed portions 61. The negative electrode collector 64 is electrically connected to a negative electrode terminal 66 attached in an insulated state with respect to a sealing body 65 that is attached to hermetically seal a mouth portion of the prismatic battery outer can 63.
A resistance welding portion 64a of the negative electrode collector 64 is made thinner than other portions. As shown in FIG. 10B, one surface of the resistance welding portion 64a of the negative electrode collector 64 is brought into abutment with a surface on one side in a portion where the negative electrode substrate exposed portions 61 are bundled. One of a pair of resistance welding electrodes 67 is brought into abutment with the surface on another surface of the resistance welding portion 64a. The other of the pair of resistance welding electrodes 67 is brought into abutment with a surface on the other side of the bunch of negative electrode substrate exposed portions 61. A current for resistance welding is caused to flow between the pair of resistance welding electrodes 67, thereby performing resistance welding. An insulation tape 68 is attached on the resistance welding portion 64a side of the negative electrode collector 64 in order to prevent a short circuit at a portion other than the welding portion 64a of the negative electrode collector 64 with the negative electrode substrate exposed portions 61. With a nonaqueous electrolyte secondary battery shown in Patent Document 1 below, a nugget 69 is formed within the bunch of negative electrode substrate exposed portions 61, and a favorable electrical connection is achieved between the bunch of negative electrode substrate exposed portions 61 and the negative electrode collector 64.
[Patent Document 1] JP-A-2010-073408
[Patent Document 2] JP-UM-A-61-016863